Off-road motorcycling has become a popular sport enjoyed by racers and riders worldwide. The most popular dirt bikes are relatively light weight motorcycles powered by a high performance 2-stroke engine. Many of the most popular models are manufactured by the Yamaha Motor Corporation of Japan (U.S. Corp. office in Cypress, Calif.).
Most notable among the Yamaha off-road models is the model YZ250, which is the bike of choice for at least sixteen different motorcycle, off-road bike, and dirt bike competitions worldwide. The YZ250 motorcycle is powered by a 249 cc liquid-cooled 2-stroke engine. The YZ250 engine has an adjustable exhaust port known as YPVS™™ (Yamaha Power Valve System). Another related model, the YZ125 engine has a split adjustable exhaust port. At low RPM the valves are closed allowing more compression and port timing producing more torque. As RPM's increase the valves are opened allowing less compression and port timing producing high RPM horsepower.